Froth Flotation
Mineral froth flotation is a known industrial process used for extracting valuable mineral content from ore obtained for example through mining. It is a surface chemistry process used to separate solids, typically fine solids, by exploiting the variation in hydrophilicity between different materials.
In a flotation cell or vessel, containing a pulp of matter such as ore from which the mineral is to be extracted mixed with liquid, air is flowed through the pulp and separation is achieved by the selective adherence of hydrophobic particles to gas bubbles whilst any hydrophilic particles remain in the liquid which flows between the gas bubbles in the vessel. When bubbles rise to the top of the vessel a froth is formed.
The froth can be arranged to overflow from the flotation vessel with both hydrophobic and hydrophilic particles comprised therein. Those particles can be extracted as a concentrate. The remaining pulp in the flotation vessel is commonly referred to as the tailings.
Cell Banks and Circuits
In practice a froth flotation plant will contain multiple cells, typically arranged in banks of similar type, where material is fed through the bank, cell by cell, and then on to the next bank. Cell types may differ between banks, the initial bank, for example, containing roughers which are used for initial crude separation of desired matter from undesired matter. Downstream, banks may include secondary roughers, also known as scavengers, which perform additional separation on the pulp which remains in a rougher after froth has been overflown therefrom. Downstream banks may also include cleaners, which perform separation on froth which has been extracted from roughers or scavengers.
Quantifying Performance
The performance quality of a flotation process can be measured with respect to two characteristics of the concentrate that is extracted from the flotation vessel—grade and recovery. Grade indicates the fraction of desired solids in the concentrate as compared to undesired solids (gangue). Recovery indicates the fraction of desired solids in the concentrate as compared to the fraction of desired solids in the original ore feed that was input into the flotation cell. A key aim of an industrial flotation process is to manipulate operating conditions in order to achieve an optimal balance between grade and recovery, with an ideal flotation process producing high recovery of high grade concentrate.
Controlling the Flotation Performance
It is known that several controllable factors can affect the performance quality of a flotation process. These include the pH of the pulp, the concentration of various chemicals added to the flotation vessel, the froth depth, solids concentration and air flow rate into the flotation vessel.
According to known methods of controlling and operating a froth flotation plant, a controller can observe a flotation cell and manually or otherwise adjust the inputs to the cell, for example by adding additional chemicals and/or changing the air flow rate into the cell, according to his or her observations. Typically these are empirical based particularly on observation of the froth surface and its behaviour. However, such methods of adjustment are often imprecise. Furthermore, changes in certain visual aspects of a flotation froth do not correspond necessarily to variation in output performance quality.
In addition, modern industrial processes make use of increasingly large flotation cells. This increase in size tends to encourage the use of increased power and air volume in flotation cells, regardless of performance considerations, increasing the inefficiency inherent in existing control and operation methods. Problems therefore remain in known practical flotation methods with respect to which variables should be observed, measured and controlled in order to optimise flotation performance, as well as how to manipulate those relevant variables accurately.
In particular, existing techniques do not provide both high grade and high recovery of the concentrate recovered from industrial flotation processes.
A discussion of investigating froth flotation performance is provided in Barbian et al, “The Froth Stability Column—Measuring Froth Stability at an Industrial Scale”, Pages 315 to 319, Centenary of Flotation Symposium, Brisbane, QLD (6-9 Jun. 2005) in which correlations are identified between a froth stability factor, air rate and froth depth in a single cell
A known technique for assessing flotation performance in a plurality of linked flotation vessels is described in Hadler, “The relationship between Froth Stability and Flotation Performance Down a Bank of Cells” (PhD thesis, University of Manchester, 2006.) Performance of the first four flotation cells of a rougher bank are cumulatively analysed. According to Hadler, the performance of linked flotation cells varies as the air addition profile, i.e. the difference in air flow rate between consecutive cells along the bank, varies. Hadler finds that a peak in stability exists in each cell in the bank as the air rate into the cell is changed. Over the range of air flow rates tested in Hadler, the cumulative grade of the concentrate decreases with increased air flow rate. Therefore low air rates and a rising air profile across the bank are employed.
In addition, the applicant is not aware of any known technique that can utilise straightforward and automatable measurement of parameters in order to reliably control operation of a group of flotation cells.
The above discussion is not to be taken as a description of the common general knowledge.